(a) Technical Field
The present invention relates to a membrane humidifier for a fuel cell. More particularly, it relates to a membrane humidifier for a fuel cell that can improve to humidification efficiency by forming a plurality of internal partition modules including a heater in the membrane humidifier and guiding humid air to the central part of a hollow fiber membrane bundle.
(b) Background Art
For the operation of a fuel cell stack, it is necessary to humidify a polymer electrolyte membrane in the fuel cell stack. Thus, a separate membrane humidifier in which exhaust gas, i.e., humid gas discharged from the fuel cell stack, and dry air, supplied from external air, exchange humidity with each other is being used.
Examples of humidifiers which are used in the present fuel cell systems include bubblers, injections, and absorbent types. Since there is a limitation in terms the size of the engine compartment in fuel cell vehicles, membrane humidifiers that have a relatively small volume and do not require a special power resources, are being utilizes in fuel cell vehicles. Particularly, membrane humidifiers using hollow fiber membranes are being suitably used because of the beneficial size and properties.
As shown in FIG. 8, in a fuel cell system, an air supply system that supplies air (oxygen) to a fuel cell stack includes a membrane humidifier. Here, a suctioning operation of an air blower 202 supplies external dry air to a membrane humidifier 100, and at the same time, exhaust gas discharged from a fuel cell stack 200 passes through the membrane humidifier 100. In this case, while humidity contained in the exhaust gas is passing through a hollow fiber membrane in the membrane humidifier 100, the dry air is humidified.
With reference to FIGS. 5 through 7, the configuration and operation of a typical membrane humidifier including a hollow fiber membrane will be described in more detail below.
Referring to FIGS. 5 through 7, a typical membrane humidifier 100 includes a housing 101. The housing 101 includes a supply port 102 for receiving dry air and a discharge port 103 for discharging humidified air. Particularly, a hollow fiber membrane bundle 107 including a plurality of hollow fiber membranes 106 is housed in the housing 101. Also, an inlet 104 for receiving humid air discharged from the fuel cell stack is formed at one side of the housing 101, and an outlet 105 for discharging humid air is formed at the opposite side of the housing 101.
Looking at the operation of a membrane humidifier using a hollow fiber membrane, when humid air, i.e., exhaust gas from a fuel cell stack is supplied from the inlet 104 of the housing 101 to the hollow fiber membrane bundle 107, humidity from the humid air is separated by a capillary action of each hollow fiber membrane 106. Here, the separated humidity is condensed while passing a capillary tube (not shown) of the hollow fiber membrane 106, and moves into the hollow fiber membrane 106. Thereafter, humidity is removed from the humid air as it moves along the outside of the hollow fiber membrane 106 to be discharged through the outlet 105 of the housing 101.
An air blower supplies external air (dry air) though the supply port 102 of the housing 101. The dry air supplied through the supply port 102 moves along the inside of the hollow fiber membrane 106. In this case, since humidity separated from the humid air has already moved to the inside of the hollow fiber membrane 106, dry air is humidified by the humidity. The humidified air is then supplied to the fuel cell stack through the discharge port 103.
However, since the hollow fiber membrane bundle 107 is a compact form of the hollow fiber membranes 106, it is difficult for the humid air introduced through the inlet 104 to penetrate into the hollow fiber membrane. Furthermore, since the diffusion speed of the humid air through the hollow fiber membrane 106 is very slow, the humid air has greater difficulty in permeating into the hollow fiber membrane 106.
Particularly, in the inside of the housing 101, the humid air passing the outside of the hollow fiber membrane bundle 107 may not permeate into the central part of the hollow fiber membrane bundle 107 that is represented as dotted lines of FIGS. 6 and 7. As shown in arrows of FIGS. 6 and 7, the humid air mostly flows along the edge of the hollow fiber membrane bundle 107. As a result, since the diffusion speed of the humid air to the central part of the hollow fiber membrane bundle 107 is very slow, the humidification efficiency with respect to dry is typically reduced. Also, since most of dry air introduced through the supply port 102 of the housing 101 flows into the central part (part represented as dotted lines of FIGS. 6 and 7) of the hollow fiber membrane bundle 107, the overall efficiency of a humidifier is even further reduced. Thus, the hollow fiber membrane 106 located at the central part of the hollow fiber membrane bundle 107 is not supplied with enough humidity, the overall efficiency of the humidifier is reduced.
This limitation can be verified in a simulation test result of FIG. 9.
That is, as shown in FIG. 9, it can be verified that most of dry air flows into only the central part of the hollow fiber membrane bundle 107.
In other words, since dry air introduced through the supply port 102 of the housing 101 mostly flows along the central part (part represented as dotted line of FIGS. 6 and 7) of the hollow fiber membrane bundle 107, and humid air introduced through the inlet 104 flows along the edge part of the hollow fiber membrane bundle 107, the humidification efficiency of the membrane humidifier is reduced. This limitation has a greater influence on the humidification efficiency of the membrane humidifier as the flow rate of the dry air increases, that is, a high power is generated from the fuel cell stack.
As describe above, the humid air supplied to the membrane humidifier is air that is discharged after a reaction in the fuel cell. Generated water as well as vapor is also supplied to the membrane humidifier together with the humid air. In cold weather, water introduced into the membrane is frozen, and thus the hollow fiber membrane fails to perform its normal function due to the freezing of water. Accordingly, the frozen water has to be melted for the operation of the membrane humidifier in cold weather. If the surface of the hollow fiber membrane is damaged by repetitive freezing and melting of the surface, the damaged hollow fiber membrane has a fatal effect on the performance of the fuel cell stack. This may incur a need of full replacement of the membrane humidifier.
The hollow fiber membrane formed of polymeric material accounts for the greatest portion of the manufacturing cost of the membrane humidifier. Since a larger amount of hollow fiber membrane bundle than is needed is used to maintain excellent humidification performance, the size of the membrane humidifier may also be relatively large compared to its actual performance.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.